The present invention generally relates to a high frequency heating arrangement and more particularly, to a driving circuit of an inverter microwave oven arranged to convert a low voltage D.C. power supply into a high frequency current of a high voltage for rectification by a voltage doubler rectifier circuit so as to feed electric power to a magnetron or the like.
Recently, there have been developed various kinds of electric and electronic appliances normally used by a commercial A.C. power source, which are capable of being used outdoors. Such appliances are required to be driven by a low voltage D.C. power supply at 12 V or 24 V or the like, for example, a storage battery for an automobile, etc. Thus, also with respect to the inverter microwave oven which is widely utilized at present, outdoor use thereof has been attempted.
FIG. 10(a) shows a general construction of a conventional typical inverter microwave oven. In this inverter microwave oven, A.C. power obtained from a commercial power source (100 V, 50/60 Hz) is converted into D.C. power by a rectifier circuit, and the D.C. power is formed into high frequency power by a one-transistor resonance type inverter circuit so as to be raised in voltage by a step-up transformer. The output of the step-up transformer is rectified by a voltage doubler rectifier circuit and is utilized for driving a magnetron.
In the case where the above inverter microwave oven is to be used by a low voltage D.C. power source, as shown in FIG. 10(b), a DC/AC inverter is provided between the low voltage D.C. power source and the inverter microwave oven so as to convert the output of the low voltage D.C. power source into the same A.C. power of 100 V, 50/60 Hz as that of the commercial A.C. power source by the DC/AC inverter for operating the inverter microwave oven by the A.C. power.
However, when the inverter microwave oven is used with the low voltage D.C. power source as described above the practice to input the A.C. power, into the inverter microwave oven by employing the DC/AC inverter, since the power conversion is effected two times at the DC/AC inverter and the inverter circuit of the inverter microwave oven, there is such a problem that the utilization rate of the electric power is extremely lowered, while the cost for the power supply circuit is undesirably increased due to the fact that two inverters are required.
Meanwhile, although it is theoretically possible to alter the arrangement to directly connect the low voltage D.C. power source to the one-transistor resonance type inverter power source circuit of the conventional inverter microwave oven, switching elements having a very large current capacity are required to cope with the lowering of the power source voltage. Such switching elements as referred to above are either not commercially available or become very expensive to be prepared.